Overview

Klinefelter syndrome is a genetic condition that results when a boy is born with an extra copy of the X chromosome. Klinefelter syndrome is a common genetic condition affecting males, and it often isn't diagnosed until adulthood.

Klinefelter syndrome may adversely affect testicular growth, resulting in smaller than normal testicles, which can lead to lower production of testosterone. The syndrome may also cause reduced muscle mass, reduced body and facial hair, and enlarged breast tissue. The effects of Klinefelter syndrome vary, and not everyone has the same signs and symptoms.

Most men with Klinefelter syndrome produce little or no sperm, but assisted reproductive procedures may make it possible for some men with Klinefelter syndrome to father children.

Symptoms

Signs and symptoms of Klinefelter syndrome vary widely among males with the disorder. Many boys with Klinefelter syndrome have few noticeable signs, and the condition may go undiagnosed until adulthood. For others, the condition has a noticeable effect on growth or appearance.

Signs and symptoms of Klinefelter syndrome also vary by age.

Babies

Signs and symptoms may include:

Weak muscles

Slow motor development — taking longer than average to sit up, crawl and walk

Delay in speaking

Quiet, docile personality

Problems at birth, such as testicles that haven't descended into the scrotum

Boys and teenagers

Signs and symptoms may include:

Taller than average stature

Longer legs, shorter torso and broader hips compared with other boys

Absent, delayed or incomplete puberty

After puberty, less muscle and less facial and body hair compared with other teens

Small, firm testicles

Small penis

Enlarged breast tissue (gynecomastia)

Weak bones

Low energy levels

Tendency to be shy and sensitive

Difficulty expressing thoughts and feelings or socializing

Problems with reading, writing, spelling or math

Men

Low sperm count or no sperm

Small testicles and penis

Low sex drive

Taller than average height

Weak bones

Decreased facial and body hair

Less muscular than normal

Enlarged breast tissue

Increased belly fat

When to see a doctor

See a doctor if you or your son has:

Slow development during infancy or boyhood. Delays in growth and development can be the first sign of a number of conditions that need treatment — including Klinefelter syndrome. Though some variation in physical and mental development is normal, it's best to check with a doctor if you have any concerns.

Male infertility. Many men with Klinefelter syndrome aren't diagnosed with infertility until they realize they're unable to father a child.

Causes

Klinefelter syndrome occurs as a result of a random error that causes a male to be born with an extra sex chromosome. It isn't an inherited condition.

Humans have 46 chromosomes, including two sex chromosomes that determine a person's sex. Females have two X sex chromosomes (XX). Males have an X and a Y sex chromosome (XY).

Klinefelter syndrome can be caused by:

One extra copy of the X chromosome in each cell (XXY), the most common cause

An extra X chromosome in some of the cells (mosaic Klinefelter syndrome), with fewer symptoms

More than one extra copy of the X chromosome, which is rare and results in a severe form

Extra copies of genes on the X chromosome can interfere with male sexual development and fertility.

Risk factors

Klinefelter syndrome stems from a random genetic event. The risk of Klinefelter syndrome isn't increased by anything a parent does or doesn't do. For older mothers, the risk is higher but only slightly.

Complications

Klinefelter syndrome may increase the risk of:

Anxiety and depression

Infertility and problems with sexual function

Weak bones (osteoporosis)

Heart and blood vessel disease

Breast cancer and certain other cancers

Lung disease

Endocrine conditions such as diabetes and hypothyroidism

Autoimmune disorders such as lupus and rheumatoid arthritis

Tooth problems that make dental cavities more likely

A number of complications caused by Klinefelter syndrome are related to low testosterone (hypogonadism). Testosterone replacement therapy reduces the risk of certain health problems, especially when therapy is started at the beginning of puberty.

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